The classical Ras, Raf, MEK (mitogen activated protein kinase/extracellular signal-regulated kinase), ERK (extracellular signal-regulated kinase) pathway plays a central role in the regulation of a variety of cellular functions dependent upon cellular context, including cellular proliferation, differentiation, survival, immortalization and angiogenesis (reviewed in Peyssonnaux and Eychene, Biology of the Cell, 2001, 93, 3-62).
In this pathway, Raf family members are recruited to the plasma membrane upon binding to guanosine triphosphate (GTP) loaded Ras resulting in the phosphorylation and activation of Raf proteins. Activated Rafs then phosphorylate and activate MEKs, which in turn phosphorylate and activate ERKs. Upon activation, ERKs translocate from the cytoplasm to the nucleus resulting in the phosphorylation and regulation of activity of transcription factors such as Elk-I and Myc. The Ras/Raf/MEK/ERK pathway has been reported to contribute to the tumorigenic phenotype by inducing immortalisation, growth factor-independent growth, insensitivity to growth-inhibitory signals, ability to invade and metastasize, by stimulating angiogenesis and by inhibiting apoptosis (reviewed in Kolch et al., Exp. Rev. Mol. Med., 2002, 25 Apr., http://www.expertreviews.org/02004386h.htm). In fact, ERK phosphorylation is enhanced in approximately 30% of all human tumours (Hoshino et al., Oncogene, 1999, 18, 813-822). This may be a result of overexpression and/or mutation of key members of the pathway.
Three Raf serine/threonine protein kinase isoforms have been reported Raf-1/c-Raf, [beta]-Raf and A-Raf (reviewed in Mercer and Pritchard, Biochim. Biophys. Acta, 2003, 1653, 25-40), the genes for which are thought to have arisen from gene duplication. All three Raf genes are expressed in most tissues but with differences: c-Raf is expressed ubiquitously at high levels, whereas [beta]-Raf high-level expression is found in neuronal tissue and A-Raf in urogenital tissue.
The highly homologous Raf family members have overlapping but distinct biochemical activities and biological functions (Hagemann and Rapp, Expt, Cell Res. 1999, 253, 34-46). Expression of all three Raf genes is required for normal murine development however both c-Raf and B-Raf are required to complete gestation. [beta]-Raf−/− mice die at E12.5 due to vascular haemorrhaging caused by increased apoptosis of endothelial cells (Wojnowski et al, Nature Genet., 1997, 16, 293-297), B-Raf is reportedly the major isoform involved in cell proliferation and the primary target of oncogenic Ras.
Activating 5 somatic missense mutations have been identified exclusively for [beta]-Raf, occurring with a frequency of 66% in malignant cutaneous melanomas (Davies et al., Nature, 2002, 417, 949-954) and also present in a wide range of human cancers, including but not limited to papillary thyroid tumours (Cohen et al., J. Natl. Cancer Inst., 2003, 95, 625-627), cholangiocarcinomas (Tannapfel et al., Gut, 2003, 52, 706-712), colon and ovarian cancers (Davies et al., Nature, 10 2002, 417, 949-954). The most frequent mutation in [beta]-Raf (80%) is a glutamic acid for valine substitution at position 600. These mutations increase the basal kinase activity of B-Raf and are thought to uncouple Raf/MEK/ERK signalling from upstream proliferation drives including Ras and growth factor receptor activation resulting in constitutive activation of ERK. Mutated B-Raf proteins are transforming in NIH3T3 cells (Davies et al., Nature, 2002, 15 417, 949-954) and melanocytes (Wellbrock et al., Cancer Res., 2004, 64, 2338-2342) and have also been shown to be essential for melanoma cell viability and transformation (Hingorani at al., Cancer Res., 2003, 63, 5198-5202). As a key driver of the Raf/MEK/ERK signalling cascade, [beta]-Raf represents a likely point of intervention in tumours dependent on this pathway.
Substituted pyrazole derivatives for the treatment of cytokine-mediated diseases such as inflammation and arthritis are disclosed in WO98/52940 and WO00/31063 in the name of G.D. Searle & Co. Hydroxyaryl-pyrazole derivatives for the treatment of cancer are disclosed in WO03/055860 in the name of Cancer Research Institute and in WO07/105,058 in the name of Pfizer Inc, Pyrimidinyl-pyrazole derivatives for the treatment of hyperproliferative disorders such as cancer are disclosed in WO07/24843 in the name of SmithKline Beecham Corporation. 3,4-Diarylpyrazole derivatives for the treatment of diseases associated by a disregulated protein activity such as cancer are disclosed in WO2010/010154. Despite these developments, there is still need for effective agents for said diseases.